Throughout this application, various publications are referenced by the last name of the authors, followed by the year of publication within parenthesis. Full citations for these publications may be found at the end of the specification, immediately preceding the claims. The disclosures of these publications are hereby incorporated by reference into this application in order to more full describe the state of the art as known to one skilled therein as of the date of the invention described and claimed herein.
In a contact-dependent process termed “T cell helper function,” CD4+ T lymphocytes direct the activation and differentiation of B lymphocytes and thereby regulate the humoral immune response by modulating the specificity, secretion and isotype-encoded functions of antibody molecules (Mitchell, et al., 1968; Michison, 1971; White, et al., 1978; Reinherz, et al. 1979; Janeway, et al. 1988; O'Brien, et al., 1988; Rahemtulla, et al., 1991; and Grusby, et al., 1991). The T cell surface molecules that mediate the contact-dependent elements of T cell helper function are not yet fully known to one (Noelle, et al., 1991).
The process by which T cells help B cells to differentiate has been divided into two distinct phases: the inductive and effector phases (Vitetta, et al., 1989; Noelle, et al., 1990). In the inductive phase, resting T cells contact antigen-primed B cells and this association allows clonotypic T cell receptor (TCR)-CD4 complexes to interact with Ia/Ag complexes on B cells (Janeway, et al., 1988; Katz, et al., 1973; Zinkernagel, 1976; Sprent, 1978a; Sprent, 1978b; Jones, et al., 1981; Julius, et al., 1982; Chestnut, et al., 1981; Rogozinski, et al., 1984). TCR/CD4 recognition of Ia/Ag results in the formation of stable T-B cognate pairs and bidirectional T and B cell activation (Sanders, et al., 1986; Snow, et al., 1983; Krusemeier, et al., 1988; Noelle, et al., 1989; Bartlett, et al., 1989; Kupfer, et al., 1987). In the effector phase, activated T cells drive B cell differentiation by secreting lymphokines (Noelle, et al., 1983; Thompson, et al., 1985) and by contact-dependent stimuli (Noelle, et al., 1989; Clement, et al., 1984; Crow, et al., 1986; Brian, 1988; Hirohata, et al., 1988; Jover, et al., 1989; Whalen, et al., 1988; Pollok, et al., 1991; Bartlett, et al., 1990), both of which are required for T cells to drive small, resting B cells to terminally differentiate into Ig secreting cells (Clement, et al., 1984; Martinez, et al., 1981; Andersson, et al., 1980).
Although the inductive phase of T cell help is Ag-dependent and MHC-restricted (Janeway, et al., 1988; Katz, et al., 1973; Zinkernagle, 1976; Sprent, 1978a; Sprent, 1978b; Jones, et al., 1981; Julius, et al., 1982; Chestnut, et al., 1981; Andersson, et al., 1980), the effector phase of T cell helper function can be Ag-independent and MHC-nonrestricted (Clement, et al., 1984; Hirohata, et al., 1988; Whalen, et al., 1988; Andersson, et al., 1980; DeFranco, et al., 1984; Julius, et al., 1988a; Julius, et al., 1988b; Riedel, et al., 1988; Owens, 1988; Cambier, et al., 1988; Tohma, et al., 1991; Lohoff, et al., 1977). An additional contrasting feature is that the inductive phase of T cell help often requires CD4 molecules and is inhibited by anti-CD4 mAb (Rogozinski, et al., 1984), whereas helper effector function does not require CD4 molecules (Friedman,e t al., 1986) and is not inhibited by anti-CD4 mAbs (Brian, 1988; Hirohata, et al., 1988; Whalen, et al., 1988; Tohma, et al., 1991). The nonspecific helper effector function is believed to be focused on specific B cell targets by the localized nature of the T-B cell interactions with antigen specific, cognate pairs (Bartlett, et al., 1989; Kupfer, et al., 1987; Poo, et al., 1988).
Although terminal B cell differentiation requires both contact- and lymphokine-mediated stimuli from T cells, intermediate stages of B cell differentiation can be induced by activated T cell surfaces in the absence of secreted factors (Crow, et al., 1986; Brian, 1988; Sekita, et al., 1988; Hodgkin, et al., 1990; Noelle, et al., 1991; Kubota, et al., 1991). These intermediate effects on B cells include induction of surface CD23 expression (Crow, et al., Jover, et al., 1989; Crow, et al., 1989), enzymes associated with cell cycle progression (Pollok, et al., 1991) and responsiveness to lymphokines (Noelle, et al., 1989; Pollok, et al., 1991; Tohma, et al., 1991; Hodgkin, et al., 1990; Noelle, et al., 1991; Kubota, et al., 1991). Although the activation-induced T cell surface molecules that direct B cell activation have not been previously identified, functional studies have characterized some features of their induction and biochemistry. First, T cells acquire the ability to stimulate B cells 4-8 h following activation (Bartlett, et al. 1990; Tohma, et al., 1991). Second, the B cell stimulatory activity associated with the surfaces of activated T cells is preserved on paraformaldehyde fixed cells (Noelle, et al., 1989; Crow, et al., 1986; Pollok, et al., 1991; Tohma, et al., 1991; Kubota, et al., 1991) and on purified membrane fragments (Hodgkin, et al., 1990; Martinez, et al., 1981). Third, the B cell stimulatory activity is sensitive to protease treatment (Noelle, et al., 1989; Sekita, et al., 1988; Hodgkin, et al., 1990). Fourth, the process of acquiring these surface active structures following T cell activation is inhibited by cyclohexamide (Tohma, et al., 1991; Hodgkin, et al., 1990). Although these studies strongly suggest the existence of activation-induced T cell surface proteins that deliver contact dependent stimuli to B cells, the molecular identities of such structures remain unknown.
The isolation of a CD4−1 Jurkat subclone (D1.1) that possessed the unique functional potential to activate B cells to express surface CD23 molecules and to support the terminal differentiation of B cells in the presence of lectins was previously reported (Yellin, et al., 1991). Jurkat D1.1 activated B cells from a large number of unrelated donors suggesting that the D1.1 effect was Ag independent and MHC unrestricted. The mechanism of Jurkat D1.1 mediated B cell activation was found to depend on cell-cell contact or close proximity because paraformaldehyde fixed D1.1 cells, but not secreted factors, possessed the ability to induce B cell CD23. In addition, the effect of D1.1 on B cells was not inhibited by anti-IL-4 antibodies. Further, the effect of D1.1 on B cells was distinct from that of IL-4 because rIL-4 but not D1.1 induced upregulation of B cell surface IgM (sIgM) (Yellin, et al., 1991; Shields, et al., 1989). Taken together, these data suggested that Jurkat D1.1 and activated CD4+ T cells shared surface structures that provide contact dependent elements of T cell help to B cells (Yellin, et al., 1991).
In this application, a murine IgG2a mAb (5c8) was generated that inhibits D1.1 mediated B cell activation and immunoprecipitates a novel 30 kilodalton (kD) non-disulfide linked protein from the surface of D1.1. On normal T cells, the 5c8 antigen is transiently expressed on activated CD4+ T cells in a manner than requires mRNA and protein synthesis. In functional studies, mAb 5c8 inhibits the ability of T cells to mediate B cell activation and terminal differentiation. Taken together, these data demonstrate that the 5c8 Ag is an important component of the activation-induced T cell surface structures that mediate contact dependent stimuli for B cell differentiation.